The growth of packet data services to support the transport of the Internet Protocol (IP) has now made packet data transmission the dominant application in telecommunication networks. This is stimulating an evolution of traditional voice centric telecommunications network designs toward more data centric designs. These data centric designs are converging toward a single packet transport technology based on Ethernet which is well suited for carrying IP traffic. Ethernet networks are now capable of supporting transport of not only packet based services, but voice and video services, allowing true convergence to a single networking technology. Support for voice services requires network timing synchronization for most applications. However, given that Ethernet is inherently an asynchronous technology, transport of timing information via Ethernet has been an issue.
Recently, the International Telecommunications Union (ITU) has developed standards for the transport of timing information via Ethernet links which have been documented in ITU-T Recommendation G.8261 as well as standards for synchronous Ethernet Equipment Clocks which have been documented in ITU-T Recommendation G.8262. These standards require that synchronous Ethernet line interfaces be synchronized to network timing sources, which then allow the lines themselves to serve as timing references. However, Ethernet signals, either synchronous or asynchronous, often need to be transported via optical transmission systems between network locations. Due to the number of Ethernet line interfaces and their bandwidths (from 10 Mbps to 10 Gbs and soon to be extended to 100 Gbps), these links are generally multiplexed together electrically before transmission over the optical network in order to improve wavelength utilization. Today this is typically accomplished by mapping the Ethernet signal into synchronous digital hierarchy (SDH) or synchronous optical network (SONET) networks, which are in turn carried over dense wavelength division multiplexed (DWDM) optical transport systems, such as those based on ITU optical transport network (OTN) standards defined in ITU-T Recommendation G.709.
In order to support synchronous Ethernet line interfaces transported via SONET/SDH networks it is required that both the Ethernet line interfaces and the SDH/SONET networks be synchronized to network timing. This is accomplished in one of two ways: synchronize the SDH/SONET network to network timing then synchronize the Ethernet line interfaces to the SDH/SONET network, or synchronize the Ethernet line interfaces to network timing then synchronize the SDH/SONET network to the Ethernet line interfaces. The first case is termed external timing from the perspective of the SDH/SONET network, while the second case is termed line timing from the SDH/SONET network perspective. Mechanisms for implementing either of these two approaches are well known and can be supported in SDH/SONET networks.
Ethernet signals are transported via the SONET/SDH network by mapping them into SONET/SDH payload containers using a mapping technique called Generic Framing Procedure (GFP). GFP is a mapping technique defined by ITU-T Recommendation G.7041. This mapping technique improves transport bandwidth efficiency by either stripping off unneeded Ethernet characters (Frame mapped GFP or GFP-F), or encoding the Ethernet stream to reduce its bandwidth while preserving all relevant payload information (transparent GFP or GFP-T). However, the GFP mapping techniques do not preserve the timing integrity of a synchronous Ethernet link due to the removal of characters or the encoding process. It is only by synchronizing the Ethernet link to the SONET/SDH timing that synchronization is preserved. This is not an issue for SONET/SDH networks because timing information is not carried through the mapping process through separate timing circuits supporting the external and line timed approaches described above.
It is also possible to carry Ethernet signals directly on OTN-based DWDM systems. OTN-based DWDM networks are asynchronous networks, that is, they are not synchronized to network timing. Therefore there is no external timing or line timing mechanism available to transport synchronous Ethernet timing information. The only standards-based mechanism for mapping Ethernet signals into OTN uses the GFP mapping technique described above, but this method does not preserve the timing integrity of the client signal.
The problem with the current state of the art is that the presence of a SDH/SONET layer is required to provide the transport of synchronous Ethernet signals along across an OTN-based DWDM network and preserve their timing integrity. It is highly desirable to be able to transport synchronous Ethernet signals directly over OTN transport systems without requiring an underlying SONET/SDH infrastructure and making network convergence to a single networking technology (Ethernet only instead of Ethernet plus SDH/SONET) possible. This is problematic since OTN-based DWDM networks are defined to operate asynchronously and there is currently no standardized mapping defined for Ethernet signals into OTN that would preserve the timing information of synchronous Ethernet line interfaces. The only standard mapping mechanism that exists within the OTN standard uses a GFP-based mapping technique which does not preserve timing information but provides better bandwidth efficiency. The only standards-based OTN client signal mapping technique that will support the preservation of timing information is a constant bit-rate (CBR) mapping mechanism, which could be applied to synchronous Ethernet client signals, but does not have the bandwidth efficiency provided by the standard GFP mapping technique.